solve #204

Author: aylei

src/lib.rs

@@ -171,3 +171,4 @@ mod n0200_number_of_islands;
 mod n0201_bitwise_and_of_numbers_range;
 mod n0202_happy_number;
 mod n0203_remove_linked_list_elements;
+mod n0204_count_primes;

src/n0204_count_primes.rs

@@ -0,0 +1,62 @@
+/**
+ * [204] Count Primes
+ *
+ * Count the number of prime numbers less than a non-negative number, n.
+ * 
+ * Example:
+ * 
+ * 
+ * Input: 10
+ * Output: 4
+ * Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.
+ * 
+ * 
+ */
+pub struct Solution {}
+
+// submission codes start here
+
+impl Solution {
+    pub fn count_primes(n: i32) -> i32 {
+        if n <= 2 { return 0 }
+        let mut is_prime = vec![true; n as usize];
+        is_prime[0] = false;
+        is_prime[1] = false;
+        let mut i = 2;
+        while i * i < n {
+            if !is_prime[i as usize] {
+                i += 1;
+                continue
+            }
+            let mut j = i * i;
+            while j < n {
+                is_prime[j as usize] = false;
+                j += i;
+            }
+            i += 1;
+        }
+        let mut count = 0;
+        for &v in is_prime.iter() {
+            if v { count += 1 }
+        }
+        count
+    }
+
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_204() {
+        assert_eq!(Solution::count_primes(10), 4);
+        assert_eq!(Solution::count_primes(2), 0);
+        assert_eq!(Solution::count_primes(3), 1);
+        assert_eq!(Solution::count_primes(5), 2);
+        assert_eq!(Solution::count_primes(1), 0);
+        assert_eq!(Solution::count_primes(120), 30);
+    }
+}